1. Field of the Invention
This invention relates to methods for preparing high strength yarns having superior yarn-to-yarn uniformity of denier, tenacity and tensile modulus and to methods for preparing uniform unidirectional fibrous sheets. Ballistic composites prepared from these yarns and fibrous sheets have improved uniformity of ballistic resistance.
The yarns and fibrous sheets are useful in applications requiring impact absorption, penetration resistance and ballistic resistance, such as body armor, helmets, breast plates, helicopter seats, spall shields; composite sports equipment such as kayaks, canoes, bicycles and boats; and in fishing line, sails, ropes, sutures and fabrics.
2. Description of the Related Art
High strength yarns for the purposes of the invention are yarns having tensile strengths determined by ASTM D2256-02 of at least 17 g/d. Examples of such yarns are aramids such as KEVLAR® brand, polybenzazoles (PBO) such as ZYLON® brand, poly {2,6-diimidazo[4,5-b4′5′-e]pyridinylene-1,4(2,5-dihydroxy)phenylene} such as M5® brand, ultra-high molecular weight poly(alpha-olefins) (UHMWPO), and their blends and mixtures. Ultra-high molecular weight polyolefins include polyethylene, polypropylene, poly(butene-1), poly(4-methyl-pentene-1), their copolymers, adducts, blends and mixtures.
High strength yarns are employed in impact resistant and ballistic resistant articles in the form of woven fabrics and also as cross-plied unidirectional sheets. Woven fabrics are preferably produced from yarns that have a small degree of twist, preferably less than about 2.5 turns/inch (0.98 turns/cm), or else are entangled as described in U.S. Pat. No. 5,773,370. Cross-plied unidirectional sheets are preferably produced from yarns that have been spread into thin layers as described in U.S. Pat. No. 4,916,000. The disclosure of U.S. Pat. No. 4,916,000 is hereby incorporated by reference to the extent not incompatible herewith.
Multi-filament high strength polyethylene yarns are commercially produced from ultra-high molecular weight polyethylene (UHMWPE) by Honeywell International Inc. Gel-spun UHMWPO fibers are prepared by spinning a solution of a UHMWPO into solution filaments, cooling the solution filaments to a gel state, then removing some or all of the spinning solvent. One or more of the solution filaments, the gel filaments and the solid filaments are drawn to a highly oriented state. The gel-spinning process discourages the formation of folded chain lamellae and favors formation of extended chain structures that more efficiently transmit tensile loads.
U.S. Pat. Nos. 4,551,296, 4,663,101, 6,448,659 and 6,969,533 and U.S. application Ser. No. 11/393,218 describe drawing all three of the solution filaments, the gel filaments and the solid filaments. A process for drawing high molecular weight polyethylene yarns is described in U.S. Pat. No. 5,741,451. Yet more recent drawing processes are described in co-pending U.S. application Ser. No. 11/206,838 and in United States Publication 20050093200. An oven useful for drawing polyethylene fibers is described in United States Patent Publication 20040040176. The disclosures of U.S. Pat. Nos. 4,551,296, 4,663,101, 5,741,451, 6,448,659, and 6,969,533, U.S. application Ser. Nos. 11/206,838 and 11/393,218 and United States Publications 20040040176 and 20050093200 are hereby incorporated by reference to the extent not incompatible herewith.
Each of these references represented an advance in the state of the art, however none suggests the process of this invention, and none satisfies all of the needs met by this invention.